The present invention relates, in general, to vertical cavity surface emitting lasers (VCSELs) and, in particular, to a system for versatile control of VCSEL polarization.
The Vertical Cavity Surface Emitting Laser (VCSEL) is rapidly becoming a workhorse technology for semiconductor optoelectronics. VCSELs can typically be used as light emission sources anywhere other laser sources (e.g., edge emitting lasers) are used and provide a number of advantages to system designers. Hence, VCSELs are emerging as the light source of choice for modern high speed, short wavelength communication systems and other high volume applications such as optical encoders, reflective/transmissive sensors and optical read/write applications. Inherently low cost of manufacture, enhanced reliability, non-astigmatic and circularly symmetric optical output are just some of the advantages of VCSELs over traditional laser sources.
However, VCSELs emit light that is polarized along their crystallographic cleavage planes. Unconstrained polarization switching is an inherent characteristic of the crystalline structure and growth process of VCSELs. As typically constructed, VCSELs can readily switch polarization states and frequently emit light simultaneously from multiple polarization states. This phenomenon presents some problems and challenges for system designers using VCSELs in their applications. Controlling polarization of the optical signal is important for many sensor applications.
Inconsistent power output and switching noise are common examples of types of problems resulting from uncontrolled VCSEL polarization. Consider, for example, an optoelectronic system that utilizes polarization selective components concentrating on only one of a VCSEL's two polarizations. If the VCSEL is emitting exclusively from that one polarization, then the system receives 100% of the VCSEL power. If, however, the VCSEL is emitting exclusively from the other polarization, then the system receives none of the VCSEL power. If the VCSEL is switching between, or emitting simultaneously from, the two polarizations, then the system will receive intermittent, or partial, power respectively. Also consider, for example, a data communications system that performs high-speed switching. Oscillation between the VCSEL polarizations results in switching noise that can impact system performance and reliability. In systems where the power and noise effects of VCSEL polarization combine, extensive noise and related system problems result.
Previously, some applications incorporated no direct solution to polarization problems; either suffering the consequences or adding a significant amount of additional circuitry and/or components to compensate for the effects. Some conventional methods have attempted to control VCSEL polarization at the chip level. These efforts have typically focused on modifying the VCSEL growth process and/or post-growth chip level processing to provide some level of polarization control. Some such approaches have introduced asymmetry to increase the degree of preference for one polarization over another. While these efforts may have, in some cases, provided partial relief from polarization problems, they often led to other problems of their own such, as increased noise and increased beam divergence. These approaches have further added to the production costs of the VCSEL components and often change the electrical characteristics of the component; yielding different reliability and new characterization data for each variation and resulting in VCSELs highly specialized for one particular application, reducing economies of scale in high volume VCSEL production.